Hi,
Check the following:
1. That the ringer volume is turned up - see p.5 of the guide, Up arrow symbol.
2. That the phones are not set for 'silent mode' - see p.11 of the User guide.
Here is a link to the user guide
https://www.uniden.com/File%20Library/FooterNav/Product%20Information/Owners%20Manuals/D1760om.pdf